Low-frequency amplifying arrangement



Oct. 2, 1951 H. A. BRoos LOW-FREQUENCY AMPLIFYING ARRANGEMENT Filed March 26, 1948 W f@- MJ AGENI a'te'nted ct. 2, T951 LOW-FREQUENCY AMPLIFYING ARRANGEMENT Henricus Adrianus Broos, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application fMarch 26, 1948, Serial No. 17,243 In the Netherlands April 8, 1947 andthe movable contact, but also with a third fixed `contact `which is connected through a highpass filter to the base of the potentiometer (the terms top and bottom `respectively of the potentiometer are to be understood to mean that, if the movable contact is located at the terminal connection top, the amplification factor 'is a maximum, whereas if it is located at the terminal connection bottom the amplification factor is a minimum). By the provision of this high-pass 4filter the low audio frequencies will relatively be emphasized with respect to the high audiofrequencies, if the movable contact of the potentiometer is located in a low position. i. e. in the proximity of the bottom of the potentiometer, since the high-pass lter provides low impedance filter path for high audio requencies. By this expedient a physiological control is effected whereby in the case of a small sound volume of a loudspeaker included in the output circuit of the amplifier, under which condition the sensitivity of the ear to bass tones falls ofi more strongly than 'that to the high-pitched sounds, the spectral sound impression remains the same.

According to the invention the circuit-arrangement lis constructed to 'be such that, if the potentiometer arm occupies a high position in the vcase vof weak input signals, the low and high *audio frequencies are ampliiied` to a lower degree than the Amedium audio frequencies, whereas, if the potentiometer occupies a low position in the event of strong input signals, the low and high audio frequencies are more strongly amplied than the medium audio frequencies` This is `schieved by inserting properly chosen tone lters between the three fixed contacts of the potentiometer referred to and two sources of positive and negative feed-back voltage proportional to the output voltage of the amplier.

The circuit-arrangements according to the invention are particularlyintended for use in the low-frequency ramplification stage of a radioreceiving set, the width of the intermediate-frequency band-pass filter of which is comparatively small. l1:35 this circuit-arrangement it is achieved that, if `a strong transmitter is being 2 received, this low-frequency amplification stage compensates the distortion which is due to this narrow intermedite-frequency band-pass lter, so that in the reception of a strong transmitter signal a high quality output voltage is realized whereas in the reception of ya weak vsignal the low-frequency amplication characteristic i curve becomes so much narrower that the interferences provoked by neighbouring transmitters are vminimized.

Receiving circuit-arrangements are `known in which the band-width of an intermediate-frequency band-pass lter is controlled automatically or not automatically, proportional. to the strength of the incoming signal. However, this band-width control, vwhich relates to the nondetected signal, has other properties than the present circuit-arrangement in which a tone (timbre) control combined with gain control is effected. More particularly the measure in which a neighbouring transmitter interferes with a received transmitter, is always adjusted to a minimum value in the present circuit-arrangement. Furthermore, with such band width controls -it is notI possible, without auxiliary means, to control the amplification of the low audio frequencies nor can this band-width control be coinbined with the gain control by the use of only one potentiometer.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference Vto the accompanying drawing, wherein Figs. 1 andz represent, by way of example, embodiments thereof, whereas Fig. 3 shows the frequency characteristic curves of the circuit-arrangement depited `in Fig. k1. The symbol pF as used throughout this specification stands for the term picofarz-id, which term is equivalent to micromicrofarad.

In Fig. 1 the tuned circuit l has fed ,to it intermediate-frequency oscillations which are detected Vby the diode detector 2. The output circuit of this diode detector 2 comprises a resistance 3 of, for example-47 kilohms and a potentiometer 4 of, say 0.8 megohm. The intermedifate-frequency oscillations are by-passed by a condenser 5.

The potentiometer 4 comprising a movable contact 6 is inserted in the input circuit of 4a discharge tube "l, in the output circuit of which oscillations vare'produced which are suppliedT if desired after amplification, to an output transformer 81to the secondary side of lwhich a loudacoger speaker 9 is connected. The secondary winding I of this transformer, is provided with connecting terminals a, b, c and d. The terminal c is connected to earth, so that between the terminals b and c a negative feed-back voltage, between the terminals c and d a positive feed-back voltage is produced.

The' bottom II of the potentiometer 3 is connected to the terminal b and thereby a negative feed-back voltage operative for al1 frequencies of the oscillations to be amplified is applied to the potentiometer. tiometer is connected to the contact d, through a medium tone lter consisting of the seriescombination of a condenser I3 and a resistance I4 passing the high medium audio frequencies. The contact I2 is similarly connected to the contact b through the series-combination of a condenser I5 and a resistance I6 passing the low medium audio frequencies. The top I1 of the potentiometer 4 is connected through resistances I4 and 20 to the terminal d. These feed-back circiuts I3, I4, 20 and I5, IS are proportioned in such manner with respect to the number of turns of the secondary winding I0 of the transformer 8 that at the top I1 of the potentiometer 4 no feed-back voltage is operative at least in regard to the medium audio range. If the movable contact of the potentiometer occupies the position I2, the frequency characteristic curve of the circuit-arrangement will, as a result thereof, exhibit a picture in which the medium audio frequencies are much more strongly fed back than the W and high audio frequencies.

If the movable contact' of the potentiometer 4 is moved towards the bottom II of the potentiometer, an ever increasing negative feed-back voltage is introduced, which again comprises medium audio frequencies for the greater part.

When the sliding contact 6 assumes the position I I the remaining signal supplied to the grid of the discharge tube 1 is negligible due to the low impedance of the winding bc.

The values of these circuit elements are, for example resistances I4 and I8=21,000 ohms resistance 20:0.47 megohm condensers I3 and I5=2200 and 12,000 pF's respectively resistance sections I1 to I2 and I2 to II==0.6

and 0.2 megohm respectively.

The circuit-arrangement so far described has some properties capable of improvement. If the sliding contact 6 is located in the proximity of the base II of the potentiometer, the high audio frequencies will be enhanced only to a low degree with respect to the medium audio frequencies, owing to the negative feed-back. In order to avoid this, the lter I8 passing high audio frequencies and consisting of a resistance of, say, 0.8 megohm and a condenser I9 of, say 20 pF, is inserted between the top I1 and the movable contact IiV of the potentiometer 4, whilst retaining the advantages of the negative feed-back.

In this position of the movable contact the low audio frequencies will, moreover, be enhanced excessively. For this purpose provision is made of a low-tone negative feed-back circuit comprising a resistance 2| which may, for example, have a value of 0.5 meg'ohm, a condenser 22 (for example 33,000 pFs.) and a resistance 23 (for example 2 megohms), the coupling condenser 24 having, for example, a value of 3300 pF.

A fixed contact I2 of this poten- The amplification characteristic curves for vrious positions of the contact 6 then exhibit in succession the forms illustrated in Fig. 3 and designated 0 dB to -50 dBs; these characteristic curves are shifted in a vertical sense over a distance which corresponds to the value of the sound strength indicated beside the characteristic curve.

By constructing the resistances 23, 25 connected to the grid of the discharge tube 1 as a potentiometer having a tapping 26, it is possible to effect sound control in a simple manner, the movable contact 21 of this potentiometer being connected, through a condenser 28 of, say 100 pF., to the terminal a of the secondary winding I0 of the transformer 8.

The negative grid-bias of the tube 1 which may.,

for instance, be derived from a by-passed resistance included in the cathode circuit of the output amplifying tube of the amplifying circuit arrangement, is supplied through the lead 29 to a resistance 30, the aforesaid condenser 22 eliminating from the amplifying tube 1 an undue backcoupling voltage of the current through this output amplifying tube on the amplifying tube 1.

Fig. 2 shows a simplified circuit-arrangement for that part of the circuit-arrangement shown in Fig. 1 which relates to the potentiometer 4. The bottom II of this potentiometer is again connected to the terminal b of the secondary winding II) of the output transformer 8, the xed contact I2 of this potentiometer 4 being connected, through the resistance 20, to the terminal d of the secondary winding I0. The part of the potentiometer 4 located between the contacts II and I2 has such a resistance as compared with the resistance 20 that the positive feed-back voltage introduced, through the resistance 20,

again compensates the negative feed-back voltage at I1. With the aid of the medium tone filter I3, I4, I5, I6 it is again achieved that, if the movable contact 6 moves towards the bottom I I of the potentiometer, the amplification characteristic curve will exhibit a form such that the low and high audio frequencies are amplified to a higher degree than the medium audio frequencies.

The circuit-arrangement is not restricted to the embodiments given by way of example, since it has been found possible for the amplification characteristic curve to be adapted in a simple manner to the frequency characteristic curve of the loudspeaker 9. Thus, for example, a comparatively large loudspeaker condenser 3I,V lfor example of 4700 pF. may be used, and the resistance 32 shown in Fig. 1, which is of the order of magnitude of 0.2 megohm permits, in a simple manner and to the correct degree, negative feedback at the resonance frequency of the loudspeaker.

Furthermore the circuit-arrangement is no restricted to the low-frequency amplification stage of a radio receiving set; it may as well be used with advantage in a gramophone amplifier, in which case it is of importance that, if the voltage set up at the output of the gramophone pickup has a low value, so that the amplification factor of the circuit must be high, the amplification characteristic curve for high audio frequencies has a lower value than for the medium audio fre-l quencies, thus reducing the needle noise.

What I claim is:

1. A circuit arrangement for amplifying signal voltages having frequency values Within a given range, comprising amplifying means having input and output terminals, a potentiometer having a bottom terminal, a top terminal, a fixed contact intermediate said top and bottom terminals and a movable contact, means to apply said signal voltages to the said top and bottom terminals of the potentiometer, means to connect said movable contact to said input terminals, means to derive from said output terminals first and second voltages, means to connect to said potentiometer said first voltage in positive feedback relationship and said second voltage in negative feedback relationship to produce in said potentiometer a point of substantially Zero feedback voltage, filter means for selectively passing low and high frequency components of said signal voltage in said given range, and means to connect said filter means to said fixed contact to produce in said potentiometer a negative feedback voltage having at medium frequencies of said range a progressively greater amplitude to- Ward said bottom terminal.

2. A circuit arrangement for amplifying signal voltages having frequency values within a given frequency range, comprising amplifying means having input and output terminals, a potentiometer having a bottom terminal, a top terminal, a fixed contact intermediate said top and bottom terminals and a movable contact, means to apply said signal voltages to the said top and bottom terminals of the potentiometer, means to connect said movable contact to said input terminals, means to derive from said output terminals first and second voltages, means to connect to said potentiometer said first voltage in positive feedback relationship and said second voltage in negative feedback relationship to produce in said potentiometer a point of substantially zero feedback voltage, filter means for selectively passing low and high frequency components of said signal voltage in said given range, means to connect said filter means to said fixed contact to produce in said potentiometer a negative feedback voltage having at medium frequencies of said ran-ge a progressively greater amplitude toward said bottom terminal, and filter means for passing high frequency components of said frequency range connected between said top terminal and said movable contact.

3. A circuit arrangement for amplifying signal i voltages having frequency values within a given frequency range, comprising amplifying means having input and output terminals, a potentiometer having a bottom terminal, a top terminal,

a fixed contact intermediate said top and bottom terminals and a movable contact, means to apply said signal voltages to the said top and bottom terminals of the potentiometer, means to connect said movable contact to said input terminals, means to derive from said output terminals first and second voltages, a first filter means for passing high and medium frequency components of said signal voltages in said given range, means to connect said first voltage through said first filter means to said fixed contact in positive feedback relationship, means to connect said second Voltage to said bottom terminal of said potentiometer in negative feedback relationship, and a second filter means for passing medium and high frequency components of said signal voltage in said given range connected between said fixed contact and said bottom terminal.

4. A circuit arrangement for amplifying signal voltages having frequency values within a given frequency range, comprising amplifying means 6 l having input and output terminals, a potentiometer having a bottom terminal, a top terminal, a fixed contact intermediate said top and bottom terminals and a movable contact, means to apply said signal voltages to the said top and bottom terminals of the potentiometer, means to connect said movable contact to said input terminals, means to derive from said output terminals first and second voltages, a common terminal for said first and second voltages, means to connect said first voltage to said fixed contact in positive feedback relationship, means to connect said second voltage to said bottom terminal of said potentiometer in negative feedback relationship, first filter means for passing high and medium frequency components of said signal voltages in said given range, second filter means for passing low and medium frequency components of said signal voltages in said given range, and means to connect said first and second filter means between said fixed contact and said common terminal.

5. A circuit arrangement for amplifying signal voltages having frequency values within a given frequency range, comprising amplifying means, a first potentiometer having a top terminal connected to said amplifying means, a bottom terminal, a fixed contact and a movable contact, a second potentiometer having a top terminal, a bottom terminal, a fixed contact intermediate said top and bottom terminals of said second potentiometer and a movable contact, means to apply said signal voltages to the top and bottom terminals of said second potentiometer, means to connect the movable contact of said second potentiometer to the fixed contact of said first potentiometer, transformer means, coupled to the output of said amplifying means and comprising a secondary Winding having a plurality of tappings, means to connect one of said tappings to ground potential, first and second resistance elements connected in series and interconnecting a second of said tappings and the top terminal of said second potentiometer, a first condenser interconnectiing the junction of said resistance elements and the fixed contact of said second potentiometer, a third resistance element interconnecting said second tapping and the fixed contact of said second potentiometer, means to connect the bottom terminal of said potentiometer to a third of said tappings, a second condenser and a fourth resistor connected in series and interconnecting the xed contact and bottom terminal of said second potentiometer, a third condenser interconnecting a fourth of said Ytappings and the movable contact of said first potentiometer, a fifth resistor element interconecting said fourth mentioned tapping and the bottom terminal of said first potentiometer, a loudspeaker element coupled to two of said tappings, and a fourth condenser shunting said two tappings.

HENRICUS ADRIAiNUS BROOS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,034,226 Carter Mar. 17, 1936 2,173,427 Scott Sept. 19, 1939 2,282,870 Lundie May 12, 1942 2,317,025 Bond Apr. 20, 1943 FOREIGN PATENTS Number Country Date 533,255 Great Britain Feb. 10, 1941 

